A radio frequency (RF) signal includes useful information that is modulated onto a carrier signal. An RF receiver retrieves the useful information from the RF signal. RF receivers are used in a wide variety of applications such as television transmission, cellular telephones, pagers, global positioning systems (GPS), cable modems, cordless phones, satellite radios, and the like. As used herein, an RF signal means an electro-magnetic signal having a frequency in a spectrum from about 3 kilohertz (kHz) to thousands of gigahertz (GHz), regardless of the medium through which such signal is conveyed. Thus an RF signal may be transmitted through air, free space, coaxial cable, fiber optic cable, etc.
In television (TV) systems, the frequency spectrum is relatively wide and is divided into separate channels that include different programming content. A TV receiver receives the wide spectrum RF signal and typically filters the wideband signal using a tracking filter tuned to the desired channel to reject unwanted signal content. The TV receiver then mixes the desired channel to a convenient intermediate frequency (IF) to make it easier to further filter the signal, and then demodulates it to recover the channel information. For example, a TV receiver designed for North American broadcasts (NTSC) translates a 6 megahertz (MHz) channel in the frequency spectrum of 48 MHz to 870 MHz to an intermediate frequency (IF) of 44 MHz for demodulation.
Not only do TV receivers require the ability to tune channels from a wideband signal, but they are often required to operate in vastly different transmission environments. For example, an over-the-air (i.e. terrestrial) TV transmission is characterized by widely varying channel signal levels resulting from different distances from fixed position broadcast antennas. To receive a terrestrial TV signal under all conditions, it is necessary to amplify potentially weak signals, and thus it is important for the receiver to maximize the signal-to-noise ratio (or conversely to minimize the noise figure (NF)). On the other hand, cable TV transmission is characterized by a large number of closely-spaced channels having uniform signal strengths. Thus while cable channels have adequate signal-to-noise ratio, the cable must be properly terminated to avoid creating large standing waves which create interference and degrade the signal. The degree of proper termination is usually measured by a parameter known as reflection loss (RL).
Historically when a user desired to adapt a terrestrial TV receiver for a cable television system, he or she typically rented a set-top-box (STB) that decoded the cable channels and remodulated them onto a particular terrestrial channel, such as channel 3 or channel 4. The STB could also conveniently be used to decode certain encrypted premium channels.
More recently it has become important for TV receivers to be able to directly tune both terrestrial and cable channels without the need for a separate set top box. The differences in the transmission environment make it difficult to design a low cost receiver which makes use of integrated circuit technology and avoids the need for costly, discrete components while still supporting both environments, due to the inherent tradeoff between good NF and good RL.
The use of the same reference symbols in different drawings indicates similar or identical items.